Electromagnetic properties of photonic devices, such as photonic crystals, have been a subject of research for decades. Of particular interest have been periodic stacks of dielectric layers. These stacked devices are typically composed of multiple repeating segments each having two or more dielectric layers with varying refractive indices. The segments are arranged in periodic manner to form a periodic stack. The spatial heterogeneity of these devices can result in qualitatively new electromagnetic properties such as strong dispersion of the group velocity of light propagating through the stack, as well as in the development of frequency gaps in the electromagnetic spectrum. These devices have found numerous applications in microwave and optical technologies.
The effects that occur when a plane electromagnetic wave is incident upon the surface of a typical semi-infinite isotropic periodic stack are well known. First, there is the possibility of omnidirectional reflection when the incident wave is totally reflected by the stack, regardless of the direction of incidence upon the stack. Second, there is the possibility of negative refraction when a tangential component of the energy flux of the transmitted wave is antiparallel to that of the incident wave. Third, there is the dramatic slowdown of the transmitted wave near a photonic band edge frequency when the normal component of the transmitted wave group velocity vanishes along with the respective component of the energy flux. All of the above effects can occur even in the simplest case of a semi-infinite periodic stack, for instance, a stack where each segment is composed of two isotropic layers, such as glass and air.
More recently, research has extended to the phenomena of electromagnetic unidirectionality in non-reciprocal magnetic photonic crystals, for instance, as discussed in A. Figotin et al., U.S. Pat. No. 6,701,048, entitled “Unidirectional Gyrotropic Photonic Crystal and Applications for the Same,” which is incorporated by reference herein as if set out in its entirety. A photonic crystal capable of electromagnetic unidirectionality is typically a periodic stack that displays strong bulk spectral asymmetry. An electromagnetic wave traveling with a given frequency in a given direction will be transmitted through the crystal, while a wave of the same frequency propagating in the opposite direction will be frozen, or will at least have a reduced or negligible group velocity. The electromagnetic wave incident on the surface of a unidirectional photonic slab gets trapped inside the magnetic photonic crystal in the form of the frozen mode with greatly reduced group velocity and greatly enhanced amplitude. These stacks are typically composed of multi-layer segments. But here, one of the layers must be made of a magnetic material with low losses and a very strong circular birefringence, or Faraday rotation. Such magnetic materials are readily available for applications at radio and microwave frequencies below 100 Gigahertz (GHz). But at higher frequencies, the magnetic materials displaying the required properties become rare, costly and problematic. In addition, all magnetic materials with sufficiently strong circular birefringence also display a very strong temporal dispersion, which may not be acceptable in a number of applications.